Method for enhancing fatigue strength of gear using shotpeening

ABSTRACT

Disclosed is a method for enhancing fatigue strength of a gear by producing compressive residual stress on a gear surface using shot peening. In the method according to the present invention, a plurality of shot balls are projected in a direction parallel to a straight line connecting a contact point of a root circle and an involute curve of a gear tooth to be subjected to the peening to a contact point of a tooth face circle and an involute curve of a gear tooth adjacent to the gear tooth to be subjected to the peening, and particularly a direction forming an angle of 0° to 15° relative to the straight line by use of high-pressure air.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for enhancing fatiguestrength of a gear by using shot peening to produce compressive residualstress on a gear surface.

[0003] 2. Description of the Related Art

[0004] In accordance with development of technologies, higher strengthis required for mechanical components used in automobiles or industrialapparatuses. To satisfy this, various method for enhancing the strengthof the mechanical components have been proposed. In a case of a gear,one of basic mechanical components, shot peening is mainly utilized inorder to enhance its strength.

[0005] The shot peening is a method for enhancing strength ofmanufactures by projecting a plurality of shot balls at a high speedonto surfaces of the manufactures to leave compressive residual stresson the surfaces. An impeller or an air nozzle is used for projecting theshot balls, between which the air nozzle grows to be more and more usedin the shot ball-projecting technology.

[0006]FIG. 1 shows a schematic illustration of a shot peening methodusing an air nozzle to project shot balls.

[0007] As shown in the drawing, a high-pressure air supply pipe 3 and ashot ball supply pipe 4 are connected to the air nozzle 2, and the airnozzle 2 faces a gear 1. Shot balls are supplied to the air nozzle 2 viathe shot ball supply pipe 4 and are projected at a high speed onto asurface of the gear 1 by means of kinetic energy of high-pressure airinjected through the air nozzle 2.

[0008] By colliding the shot balls at a high speed against the surfaceof the gear 1, compressive residual stress on the gear surface isgetting larger by impact due to the collision. As a result of this,strength of the gear 1 is enhanced, and particularly strength of a rootportion of tooth to which force is applied upon operation of the gear 1is greatly increased, thereby making it possible to prevent breakage ofthe gear 1.

[0009] In such a conventional method for enhancing strength of the gear1 using the shot peening, the shot balls is projected from the airnozzle 2 toward a center of the gear 1. In other words, the shot ballsare projected in a direction orthogonal to a tangent line of a gearcircle such as a root circle or a pitch circle. Consequently, thecompressive residual stress produced on a side surface of the gear toothsloping to the direction of projection is weaker than that produced on atooth face or a tooth flank.

[0010] As shown in FIG. 2, that is, whereas impact energy E₁ appliedupon collision of the shot balls against the tooth face or the tooth hasa magnitude of ½·mv² because the shot balls are projected in theorthogonal direction to the tooth face or the tooth flank, impact energyE₂ applied upon collision of the shot balls against the side surface ofthe gear tooth has a magnitude of ½·mv² sinθ because the shot balls areprojected onto the side surface inclined to the tooth face or the toothflank at an angle of inclination (θ). By this reason, the compressiveresidual stress produced on the side surface of the gear tooth is weakerthan that produced on the tooth face or the tooth flank. Due todeformation caused by this difference in the compressive residualstress, protrusions are formed at both ends of the tooth faces, i.e.,outer edges of the gear tooth.

[0011] Since such protrusions formed on the outer edges of the geartooth repeatedly interferes with and impacts on the tooth face of thegear during operation of the gear, there is a problem in that the toothface of the gear is broken down and thus life span of the gear isshortened.

[0012] To prevent this, the protrusions must be removed using asemi-topping or a grinding process followed by the shot peening processduring manufacture of the gear. This means that additional equipmentsand workers are required for removing the protrusion, which causesincrease of production cost and lowering in productivity.

[0013] On the other hand, force is most applied to the side surface ofthe gear tooth when the gear is in operation. Nevertheless, becausemagnitude of the compressive residual stress produced on the sidesurface of the gear tooth is smaller than that produced on the toothface to which force is less applied than to the side surface, processtime required for the shot peening must be extended so as to impartnecessary level of strength to the side surface of the gear.Accordingly, there is another problem in that working hours and quantityof consumption of the shot balls are increased and life span of thepeening device is shortened.

SUMMARY OF THE INVENTION

[0014] Accordingly, the present invention has been made to overcome theabove-mentioned problems, and it is an object of the present inventionto provide a method for enhancing fatigue strength of a gear using shotpeening, in which magnitude of compressive residual stress produced on aside surface of a gear tooth is larger at least than that produced on atooth face so that formation of protrusions at outer edges of the geartooth can be prevented and process time required for the shot peeningcan be shortened.

[0015] To achieve this object, there is provided a method for enhancingfatigue strength of a gear using shot peening in accordance with thepresent invention, in which:

[0016] a plurality of shot balls are projected onto a gear surface in adirection parallel to a straight line connecting a contact point of aroot circle and an involute curve of a gear tooth to be subjected to thepeening to a contact point of a tooth face circle and an involute curveof a gear tooth adjacent to the gear tooth to be subjected to thepeening by use of high-pressure air.

[0017] Preferably, the direction of projection of the shot balls formsan angle of 0°to 15°relative to the straight line connecting the contactpoint of the root circle and the involute curve of the gear tooth to besubjected to peening to the contact point of the tooth face circle andthe involute curve of the gear tooth adjacent to the gear tooth to besubjected to the peening.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above objects, and other features and advantages of thepresent invention will become more apparent from the following detaileddescription in conjunction with the drawings, in which:

[0019]FIG. 1 is a schematic view illustrating a conventional method forenhancing fatigue strength of a gear using air nozzle-shot peening;

[0020]FIG. 2 is a view showing a direction of projection of shot ballsin the conventional method;

[0021]FIGS. 3 and 4 are views for explaining processes of setting thedirection of projection of the shot balls in a method for enhancingfatigue strength of a gear using shot peening according to the presentinvention, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Hereinafter, a preferred embodiment of method for enhancingfatigue strength of a gear using shot peening according to the presentinvention will be described with reference to the accompanying drawings.

[0023] The present inventors have conducted a series of experiments forfinding out relationship between an angle at which shot balls areprojected onto a working surface during the shot peening and compressiveresidual stress produced on the working surface. The experiments areperformed in such a manner that the shot balls (round cut wire type, 0.6mm in diameter, HRC 58˜62) are projected onto a flat carburizing steelplate (texture composition: martensite 80 %, austenite 20 %) atdifferent angles of projection and then magnitude and depth of thecompressive residual stress produced on a surface of the steel plate aremeasured for each angle. Results of the experiments are shown in thefollowing Table. Angle of projection of shot balls 90° 80° 70° 60° 50°Maximum compressive 135 130 111 105 101 residual stress (kg/mm²) Depthof compressive 120  95  88  78  75 residual stress produced (μm)

[0024] It can be seen from the Table that magnitude and depth of thecompressive residual stress produced on the working surface is thelargest when the shot balls are projected in a direction orthogonal tothe working surface, and decreases more and more as the angle ofprojection grows smaller. This can be also theoretically proved from theequation of kinetic energy, ½·mv² sinθ.

[0025] On the basis of the above results, in the inventive method forenhancing fatigue strength of a gear using shot peening, the shot ballsare projected during the shot peening process in a direction making anangle βat which the shot balls are projected onto a side surface of agear tooth, that is, a portion to be subjected to large force when thegear is in operation be larger than an angle a at which the shot ballsare projected onto a tooth face or a tooth flank, that is, a portion tobe subjected to relatively small force as shown in FIG. 3.

[0026] As a consequence of this, magnitude of the compressive residualstress produced on the side surface of the gear tooth becomes similar toor larger than that produced on the tooth face. Accordingly, it ispossible to prevent deformation of outer edges of the gear tooth andthus formation of protrusions at outer edges of the gear tooth.

[0027] From another point of view, it is required to increase fatiguestrength of a portion of the side surface of the gear tooth to which thelargest force is applied when the gear is in operation, that is, adeddendum point B2. Taking consideration into this, optimal direction ofprojection is theoretically said to be a direction parallel to astraight line connecting a contact point B2 of a root circle and aninvolute curve of a gear tooth to be subjected to the peening to acontact point B1 of a tooth face circle and an involute curve of a geartooth adjacent to the gear tooth to be subjected to the peening as shownin FIG. 4. If the shot balls are projected in this direction, thefatigue strength of the portion of the gear tooth to which the largestforce is applied can become the largest.

[0028] In practice, however, there may occur a phenomenon that a part ofthe projected shot balls change their moving direction due to collidingagainst the point B2 and so interrupt a path of the shot balls movingtoward the point B2. In order to prevent this phenomenon, the shot ballsmust be projected in a direction in which bad influence on the path ofthe shot balls projected toward the point B2 can be minimized.

[0029] Although such a direction is determined by various factors suchas size and hardness of the shot balls, projection speed, shape of themanufacture and so on, several experiments have exhibit that thecompressive residual stress produced at the deddendum point B2 to whichthe largest force is applied has a maximal magnitude in mostmanufacturing specifications of the shot ball and the gear when the shotballs are projected in a direction forming an angle of 0°to 15°relativeto the straight line connecting the point B2 to the point B1.

[0030] In the end, by projecting the shot balls in the direction formingan angle of 0°to 15°relative to the straight line connecting the pointB2 to the point B1, the magnitude of the compressive residual stressproduced on the side surface of the gear tooth comes to be similar to orlarger than that produced on the tooth face. Accordingly, the formationof the protrusions at outer edges of the gear tooth as in theconventional method can be prevented, and so the semi-topping orgrinding process for removing the protrusions does not have to beperformed.

[0031] Moreover, the necessary time to obtain the fatigue strengthrequired for the side surface of the gear tooth can be shortened becausethe magnitude of the compressive residual stress produced on the sidesurface of the gear tooth, in particular, the deddendum point B2 ismaximized by projecting the shot balls in the above-mentioned direction.

[0032] While the present invention has been illustrated and describedunder considering a preferred specific embodiment thereof, it will beeasily understood by those skilled in the art that the present inventionis not limited to the specific embodiment, and various changes,modifications and equivalents may be made without departing from thetrue scope of the present invention.

What is claimed is:
 1. A method for enhancing fatigue strength of a gear using shot peening which produces compressive residual stress on a gear surface by projecting a plurality of shot balls onto the gear surface by use of high-pressure air, in which: the plurality of shot balls are projected in a direction parallel to a straight line connecting a contact point of a root circle and an involute curve of a gear tooth to be subjected to the peening to a contact point of a tooth face circle and an involute curve of a gear tooth adjacent to the gear tooth to be subjected to the peening.
 2. A method as recited in claim 1, wherein the direction of projection of the shot balls forms an angle of 0°to 15°relative to the straight line. 